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Wood degradation in the digestive tract of the formosan subterranean termite (Isoptera: Rhinotermitidae).

INTRODUCTION

Most wood decomposition in the digestive tract of subterranean termite workers (family Rhinotermitidae) occurs in food vacuoles of flagellate protozoan symbionts in the hindgut (Radek, 1999). Types of protozoa vary between termite species. Formosan subterranean termites (Coptotermes formosanus Shiraki) harbor three different flagellates, each occupying a different niche in the hindgut (Lai et al., 1983). Pseudotrichonympha grassi, the largest, engulfs wood particles and digests cellulose and other polysaccharides (Yoshimura, 1995). Hodomastigitoides hartmanni also engulfs wood, but appears to have different dietary requirements than P. grassi, which is not able to digest low molecular weight cellulose (Yoshimura, 1995). Spirotrichonympha leidyi, a much smaller species, obtains dissolved nutrients from gut fluid (Yoshimura, 1995). Some wood carbohydrate may also be digested before reaching the hindgut by enzymes produced in termite tissue (Radek, 1999). The current study aimed use scanning electron microscopy to analyze the degree of degradation of wood particles in different regions of C. formosanus gut.

MATERIALS AND METHODS

Coptotermes formosanus workers used for this study were from a colony collected at Audobon Park in New Orleans, LA. Insects were kept in polystyrene containers (Tri-State Plastics, Latonia, KY) containing weathered yellow pine blocks, approximately 20 cm x 4 cm x 1 cm, at 28[degrees] C and saturated humidity. Dissection of whole guts was carried out as described in Zhou et al. (2007). Excised guts were transferred to a piece of nonstick aluminum foil placed on dry ice. A double-edged razor blade was used to subdivide guts into foregut, midgut, forward hindgut, middle hindgut, and rear hindgut as illustrated in Lai et al. (1983). Quick cutting with the blade sealed gut sections containing fluid with ingested wood particles. Three sets of 10 foregut and midgut sections, and three sets of five hindgut sections, were gently squeezed with forceps in micro-centrifuge tubes containing distilled water. Tubes were stored overnight at -20[degrees] C before further processing. To remove salts in the gut fluid that would coat wood particles during drying, thawed samples were centrifuged twice for 5 minutes at 7,000 g with one change of distilled water, followed by 5 minutes at 20,800 g after an additional change of water. Tubes were vortexed and two drops immediately transferred with a disposable pipette to scanning electron microscope stubs. Stubs had a piece of carbon mounting tape affixed, with a grid pattern cut onto the tape with a razor blade. Squares of grids were approximately 0.5 [mm.sup.2]. Stubs were air dried under a fume hood, sputter coated, and wood particles viewed with a Zeiss EVO-40VXP environmental electron microscope. Micrographs were obtained at 2,000 x magnification for five different areas within a square on the grid. This was repeated for five different squares of the grid.

Degree of wood degradation was determined from comparing the number of apparent holes in wood particles recovered from each gut section. Holes were defined as fully enclosed areas on a wood particle darker than the surrounding particle surface and longer than 10 um. One-way ANOVA and Tukeys HSD was carried out using SAS software (SAS Institute) to identify significant differences in numbers of holes in wood particles between gut regions.

RESULTS

Electron micrographs showed a variety of shapes and sizes of wood particles recovered from all gut sections (Figures 1-5). The Tukey's HSD test shows wood from the middle and rear hindgut was significantly more degraded than particles from the fron foregut and midgut (Figure 6).

DISCUSSION

Fluid from insect hindgut can generally be regurgitated to the midgut and foregut. For termites, however, this is not possible as the enteric valve prevents backflow to the midgut (Noirot and Noirot-Timothee, 1969). Wood particles in the hindgut would therefore be expected to be much more degraded compared to forward gut sections. This was apparent from significantly higher numbers of holes in wood recovered from the middle and rear sections of the hindgut. More P. grassi and H. hartmanni are found in combination at the front of the hindgut compared with the middle and rear sections (Lai et al., 1983). Wood degradation at the front hindgut from its digestion by these symbionts could explain the significantly higher numbers of holes in particles from middle and rear hindgut sections. Although some wood is theoretically digested from the action of endocellulase and beta glucosidase released from termite salivary glands and midgut epithelial cells (Slaytor, 2000), numbers of holes were not significantly different except in the hindgut, Figure 6), indicating little digestion had occurred in foregut and midgut. Different explanations of how wood is digested in lower termite digestive tract have been published. Some authors, such as Slaytor (200) and Yoshimura 91995), submit cellulolytic enzymes produced in termite tissue degrade woor in forward gut sections. However, results of the current study indicate wood decomposition chiefly occurs in the hindgut. This supports the long-held conclusion that wood digestion in lower termites is carried out maily by masses of symbiotic protozoa that inhabit the hindgut (Noirot and Nirottimothee, 1969)

LITERATURE CITED

Lai, P.Y., Tamashiro, M., and Fujii, J.K. 1983. Abundance and distribution of the three species of symbiotic protozoa in the hindgut of C. formosanus. Proceedings of the Hawaiian Entomological Society 24, 271-276.

Noirot, C., and Noirot-Timothee, C. 1969. The digestive system. In: Biology of Termites, vol. 1. Krishna, K., and F.M. Weesner [eds.] Academic Press. NY.

Radek, R. 1999. Flagellates, bacteria, and fungi associated with termites: diversity and function in nutrition --a review. Ecotropica 5: 183-196.

Slaytor, M. 2000. Energy metabolism in the termite and its gut microbiota. In: Termites: Evolution, Sociality, Symbioses, Ecology. Abe, T., D.E. Bignell, and M. Higashi [eds.] Kluwer Academic Publishers. Boston.

Yoshimura, T. 1995. Contribution of the protozoan fauna to nutritional physiology of the lower termite, Coptotermes formosanus Shiraki (Isoptera: Rhinotermitidae). Wood Research 82: 68-129.

Zhou, X., Smith, J.A., Koehler, P.G., Oi, F.M., Bennett, G.W., and Scharf, M.E. 2007. Correlation of cellulase gene expression and cellulolytic activity throughout the gut of the termite Reticulitermes flavipes. Gene 395: 29-39.

Tim J. Arquette (1, 2), Amanda M. Lawrence (2, 3), Blair J. Sampson (4), and Jose M. Rodriguez (5)

(1) Mississippi State University Formosan Termite Research Laboratory, Poplarville, MS

(2) Mississippi State University Department of Biochemistry, Molecular Biology, Entomology, and Plant Pathology, Mississippi State, MS

(3) Mississippi State University Electron Microscope Center, Mississippi State, MS

(4) USDA Agricultural Research Service, Poplarville, MS

(5) State Chemical Laboratory, Mississippi State, MS

Corresponding author: Tim Arquette tia68@msstate.edu
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Article Details
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Author:Arquette, Tim J.; Lawrence, Amanda M.; Sampson, Blair J.; Rodriguez, and Jose M.
Publication:Journal of the Mississippi Academy of Sciences
Article Type:Report
Geographic Code:1USA
Date:Apr 1, 2013
Words:1078
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